1. Field of the Invention
This invention pertains to novel aqueous acid compositions comprising (a) hydroxyethylethylenediaminetriacetic acid (HEDTA), and (b) a compatible acid corrosion inhibitor. This invention also pertains to a method of using such compositions to chemically clean (remove) iron oxide scale from metal surfaces and a method of passivating the clean surface against corrosion.
2. Technology Review
The invention utilizes an organic polycarboxylic acid referred to a hydroxyethylethylenediaminetriacetic acid (abbreviated as HEDTA). This known compound corresponds to the structural formula: ##STR1## HEDTA is a solid having a melting point of 159.degree. C. and it is soluble in both water and methanol. The ammonium and alkali metal salts of HEDTA are also known.
HEDTA has been used in certain instances as a chelant. The ammoniated or aminated salts of HEDTA have also been used as chelants in removing scale from metal surfaces and for passivating ferrous metal surfaces. These salts were said to be effective against water hardness type scale (i.e. predominantly calcium and/or magnesium salts, such as calcium sulfate, calcium carbonate, etc.) and scales containing a high iron oxide content. See. U.S. Pat. No. 3,308,065 (Lesinski).
A wide variety of other organic polycarboxylic acid have also been used in chemical cleaning and/or for passivating ferrous metal surfaces. The following printed publications are known to Applicant and generally represent the state of the art:
______________________________________ U.S. Pat. Nos. 3,072,502 3,595,799 3,308,065 3,627,687 3,413,160 3,639,279 3,438,811 3,668,009 3,438,901 3,684,720 3,492,238 3,806,459 3 510,351 3,510,432 3,547,697 Japanese Patents British Patents J5 0,022,721 1,518,321 J5 0,030,928 1,182,247 J5 3,125,937 J7 4,014,629 J7 8,044,895 USSR Belgium 309,072 740,608 567,080 803,097 West Germany 2,054,067 ______________________________________
See also: NACE Corrosion '978, Atlanta, Ga. (1978): Papers 38 and 208.
In other instances, organic acids containing acid groups other than carboxylic acid groups have been presented as mimics of polyalkylenepolycarboxylic acid chelants. See, for example, U.S. Pat. No. 3,996,062 were polyalkylenepolyphosphonic acids (and alkali metal or amine salts thereof) are described. It is not known whether or not such systems have been commercialized.
But, returning to the more relevant art, a review of the above patents shown that a variety of ammoniated or aminated polyalkylenepolycarboxylic acids have been described as useful chelants for chemical cleaning. HEDTA is one of the acids named. But this review also indicates that when such compounds are used, the pH is preferably weakly acidic or basic, preferably basic. The use of ammoniated ethylenediaminetetraacetic acid at pH of from about 8.5 to about 10 (as per U.S. Pat. No. 3,308,065, U.S. Pat. No. 3,413,160 and/or U.S. Pat. No. 3,438,811) continues to represent the state of the art from a commercial standpoint.
To applicants knowledge, the prior art does not teach or suggest the invention now described.